1. Field of the Invention
The present invention relates to a process and device for monitoring the transmission of information between the central unit of a programmable controller and the input/output circuits connected to the sensors and/or to the actuators of the process controlled by said controller. It also relates to a process for updating an input image memory with respect to the inputs, and of the outputs with respect to an output image memory.
2. Description of the Prior Art
Generally, it is known that a programmable controller uses a processer for ensuring the ordered control of the actuators of the process as a function of the information supplied by the detectors, and this according to an operating sequence determined by a program.
For providing its connection with the process, the controller comprises a plurality of connecting elements broken down into two assemblies: namely:
an assembly of input connection elements to which the detectors or the sensors are connected, and
an assembly of output connection elements to which the actuators of the process are connected.
The processor conventionally comprises a central unit, usually with microprocessor, in which the transfer of information takes place through buses (data bus, address bus, control bus). This central unit communicates with the outside and in particular with the input/output connection elements of the controller by means of input/output circuits connected to the buses generated by the microprocessor.
Generally, these input/output circuits are formed on one or more printed circuit boards each combining a predetermined number of input and/or output channels.
Each of these channels ends in a multipole input or output materialized by an area for connection by poles to the printed circuit. It will be noted in this connection that twin pole inputs and outputs are very often used and accessorily three pole outputs (AC outputs).
The connection areas may be advantageously disposed along a border of the printed circuit, on which border a terminal strip is connected, for example of the type described in the Republic of France patent Published under No. 2 544 556 on Oct. 19, 1984 in the name of the applicant. This terminal strip is more particularly intended for providing a disconnectable connection between said areas and the corresponding conductors connecting the controller to the detectors and to the actuators of the process.
Thus, in a controller of the kind previously described, the aim of the invention is first of all to provide a device for detecting anomalies in the transfer and processing of information between the central unit on the one hand, and the input/out circuits connected to the detectors and/or to the actuators, on the other.
To obtain these results, it has already been proposed, in the general case of connections between computers and peripherals, to effect the transmission of a parity bit associated with each word, by providing a repetition procedure when an error is discovered. However, this solution does not give a satisfactory result in systems having timing constraints similar to that of the limited cycle time that a programmable controller must comply with for the cyclic refreshment of its input/outputs (this cycle time being of the order of a few milliseconds). Moreover, in the field of programmable controllers, when the transmission system between the central unit and the inputs/outputs comprises safety devices, they consist either in repeating each exchange twice and validating it at each identical transmission, or in checking the correct operation of the connections by inserting test exchanges between the useful exchange cycles.
With test exchanges inserted between the useful cycles, it can be checked whether the connection between the central unit and the inputs/outputs operates well under certain circumstances. However, such a cyclic check of proper or improper operation only reveals permanent faults, but does not prevent some instantaneous disturbances such as parasites from affecting the useful transmission cycles.
The procedure which consists in repeating each exchange and in validating it when there is identity, has the advantage of considerably increasing the cycle time for refreshing the inputs/outputs.
In addition, it does not take into account the fact that, in the case of the controller, little information changes from one cycle to the other, which allows the information transmitted during the preceding cycle to be used for an additional cycle without major disadvantage for the correct monitoring of the automatic device concerned